Vehicles may include various types of powertrains, including engine-based, electric, and hybrid powertrains. In some electric or hybrid vehicles, vehicle braking uses a larger amount of battery energy than any other vehicle function. Some vehicles include regenerative braking systems configured to store kinetic energy produced during vehicle braking. While electric motors are capable of producing braking torque and returning power to vehicle batteries, the batteries often are inefficient at high charge rates and cannot always accept the required power levels when fully charged. Consequently, either the regenerative braking levels are set very low, or a load dump component (e.g., large resistor) and control are added to dissipate the extra power as heat.
Some systems use ultra-capacitors or a high power battery technology in parallel with a conventional battery pack, often resulting in higher vehicle cost. In some systems, such an arrangement locks the state of charge of the two battery packs together, so that both are fully charged at the same time. It has also been proposed to use a DC-DC converter to decouple the ultra-capacitors from the battery, which introduces an additional power conversion between the motor and the energy storage.
Electric vehicles often include on-board battery chargers in the 800 to 3000 watt range or other suitable power ranges. Electric vehicles also often require 12 volt auxiliary power for lighting, fans, controls, and other accessories. In some electric vehicles, this auxiliary power is generated by a DC-DC converter that converts power at the battery pack voltage to power at the auxiliary power level (e.g., 12 VDC). The DC-DC converter is sized to supply the peak power requirements, which may be high for some electrical devices such as headlights, fans, winches, etc. If a DC-DC converter's output is exceeded during vehicle operation, the output voltage may collapse causing relays to drop out and other devices to malfunction. For loads with very high peak requirements, such as winches for example, some vehicles include an additional battery to provide adequate power to the large load. In some vehicles, the auxiliary loads are powered directly from one of the batteries making up the high voltage battery pack, typically the lowest potential or “bottom” battery, i.e., the battery whose negative terminal is connected directly to ground. This often leads to over-discharge and/or undercharge of the lowest potential battery and overcharge of the remainder of the battery pack, potentially reducing battery life.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.